1. Field of the Invention
This invention relates to a radiation image information read out device for stimulating a stimulable phosphor carrying a latent image of a radiation image in the form of stored energy of radiation and causing the phosphor to emit light according to the stored energy, and more particularly to a device for controlling the gain of a radiation image information read out device.
2. Description of the Prior Art
When a stimulable phosphor is exposed to a radiation like X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays and ultraviolet rays, a part of the energy of the radiation is stored in the phosphor. Then, when the stimulable phosphor is subjected to exposure to stimulating rays, the phosphor emits light according to the amount of energy stored therein.
A radiation image recording system using the stimulable phosphor in which a radiation image of a human body or the like is first recorded in a stimulable phosphor in the form of a pattern of energy of radiation, the stimulable phosphor carrying the radiation latent image is exposed to a laser beam or other stimulating rays to emit light according to the stored energy, the emitted light is detected by photodetectors and the detected information is converted to an image signal and used for modulating a recording light beam to record a radiation image on a photosensitive material like a photographic film is proposed as disclosed in U.S. Pat. No. 3,859,527.
In the above described radiation image recording system, there is a problem in the step of reading out the recorded radiation image information that the intensity of the light emitted from the stimulable phosphor greatly changes even if the intensity of the stimulating laser beam is maintained constant because the image recording conditions such as the dose of the radiation used for recording the image in the phosphor, the spectrum of the radiation energy and the kind of the object are not always the same. Accordingly, the radiation image finally obtained does not have stable quality. In other words, the contrast or density of the finally obtained image changes for every image according to the difference in the image recording conditions. Therefore, in case of fixing the gain of the image read out device, even if the gain of the image read out device is proper for one radiation image, it may be improper for another radiation image because the gain is too small to provide a properly high signal-to-noise ratio.
Further, the output of the laser source for emitting a laser beam as the stimulating rays is unstable and fluctuates or decays by 10 to 20% during its operation. Since the intensity of the light emitted from the stimulable phosphor is nearly in proportion to the intensity of the stimulating rays, the fluctuation or the decay of the output of the laser source provides a serious problem in obtaining a radiation image of high quality and desirable contrast and density.
Therefore, there is a demand for a radiation image information read out device in which the gain of the read out system is controllable according to the image recording condition and the level of the power of the stimulating ray source.